The capability of determining shaft rotational speed in quite short time periods has several evident advantages, the primary of which is the ability to quick-correct departures of shaft rotational speed from a desired datum level, e.g., within several seconds.
Such quick-correct ability can readily be obtained by providing for direct discernment of shaft speed in a short interval. Typically, one provides a speed wheel having sixty teeth and rotative with the shaft. The teeth are sensed magnetically by a transducer which yields a discrete electrical signal upon the passage of each tooth through its field of view or sensitivity. The shaft revolutions per minute (rpm) can be known in a one second interval. Thus, if five teeth are observed by the transducer in one second, an electrical counter will be stepped accordingly to a five count. A sixty multiple thereof will produce output indication of three hundred rpm.
In various situations, space limitations preclude the use of the relatively bulky sixty-tooth speed wheel. Here, one looks to a single slug mounted on the shaft and functioning as a one-tooth speed wheel. As the transducer now can provide not more than one pulse per revolution, transducer pulses are counted over an expanded time period, e.g., one minute. This practice has serious disadvantages over that first-discussed. The one-minute time period which must pass before speed can be read is too time-consuming for various applications requiring alteration of rpm for different test conditions.
Further, one cannot accurately read rpm until two update cycles following introduction of a change in motor speed. By way of example, assume a motor shaft to be rotating at one hundred and fifty rpm and that its speed is to be changed to one hundred and eighty-five rpm. As adjustment is made to the speed setter, say a rotary dial, motor speed increases as motor drive current increases. During the overall time period, i.e., preadjustment, adjustment and post-adjustment, the transducer output signals being counted occur at various motor speeds--initially at one hundred and fifty rpm, next at a speed between one hundred and fifty rpm and one hundred and eighty rpm, and ultimately at a speed approaching or somewhat exceeding one hundred and eighty rpm. After a one minute period, the counter will indicate perhaps one hundred and fifty-eight rpm. Clearly, the counter output cannot be the actual shaft speed at the time of readout, since the counter input during the minute interval preceding readout is provided in part at shaft speeds less than such actual shaft speed at readout. Another cycle is required to accurately read out actual speed, and if actual shaft speed is not precisely the desired one hundred and eighty rpm, one or more further cycles are required. Evidently, this is an intolerable situation for applications in which shaft speed is to be changed accurately and quickly.